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08/01/2008

Dual-function Underfills Offer Versatility

BY BRUCE CHAN, ROBERT CHU, and BRIAN TOLENO, Ph.D., The Electronics Group of Henkel

Underfills enable numerous package designs and provide the required support and reliability needed for highly miniaturized and lead-free devices. Without these essential materials, many of today’s advances would not be possible. Developments in underfill technology such as enhancements in filler technology, better control of flow rates, new cure mechanisms, improved modulus properties, and alternative application techniques have brought enhanced performance capabilities to the market. But as the industry continues its march forward toward more efficient, flexible and miniaturized devices and component configurations, more underfill system capabilities will be required.

To date, the four most commonly used types of underfills are capillary flow materials, fluxing (often referred to as no-flow) underfills, cornerbond, and edgebond systems. Each has relevance for certain applications, but newer devices — and even some older-generation packages — may benefit from a breakthrough underfill material technology in the reflow-cured encapsulant class. This epoxy flux system enables many applications in both semiconductor packaging and PCB assembly, as well as some of the emerging device configurations such as package-on-package (PoP).

Figure 1. Reflow curable underfills offer throughput advantages.

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Epoxy Flux Underfill Technology

Designed to offer process efficiency, epoxy flux underfills deliver a fluxing component that facilitates solder joint formation as well as an epoxy system that offers added device protection by encapsulating individual bumps. Because epoxy fluxes are cured during the reflow process, they offer an in-line alternative to other underfill mechanisms, eliminating the need for a dedicated dispensing system as well as the time required to dispense and cure (Figure 1). These underfill systems also provide deposition flexibility and, depending on the application and process, can be screen printed, dipped, jetted, or dispensed as required. While there are certainly other fluxing ??? or no-flow ??? underfill materials that offer in-line processing, none deliver the processability of epoxy fluxes. For example, no-flow underfill encapsulants, have been used in both semiconductor packaging and PCB assembly and, although process efficient, challenges with performance and reliability exist. Using the no-flow technique, material is applied to the substrate prior to component or die placement, and then cured during reflow. However, since moisture outgassing from the substrates and packages into the no-flow material causes voids, many packaging and assembly specialists have migrated to reflow-cured cornerbond or edgebond materials that do not fully underfill the device, or traditional capillary flow materials. Epoxy fluxes, on the other hand, only encapsulate individual spheres or bumps, leaving channels underneath the device that allow volatile gasses from the substrate to escape, while still providing solder joint protection.

Ball Attach

From water-washable to no-clean, there are countless tacky flux formulations used for solder-ball attach, each with unique features and benefits. Epoxy flux, however, may prove to be the most effective attachment method from a reliability standpoint. A study was conducted to test the shear strength of four flux types to evaluate the most robust solder sphere attachment mechanism. Three solder sphere alloys (all SAC variants) were used: SAC-1, SAC-2, and SAC-3. The shear strength of each solder sphere alloy was tested against four different flux types: two water-washable fluxes (flux A and flux B), a no-clean flux (flux C), and an epoxy flux (flux D). The flux was dispensed as single drops on the copper coupon, and the balls were deposited individually by a ball dispenser, which picks up the ball by suction and places it onto the dispensed flux. Using the single-ball shear test at a shear height of 30 µm and a shear speed of 0.5-mm per second, each material combination was evaluated.

Figure 2. Various flux performance with SAC-1.

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With each of the three alloys, the epoxy flux material delivered the strongest solder joint as compared to the other three fluxes being tested. (Figures 2-4) These results suggest that higher reliability can be achieved by using an epoxy flux material for ball attach than by using traditional flux formulations.

PoP Configurations

Like ball-attach processes, epoxy fluxes are advantageous for emerging PoP device configurations. While PoP devices offer improved efficiency by maximizing PCB or substrate real estate, there are challenges with the second-level assembly of these packages. Bottom-level package assembly is straightforward and follows standard surface-mount procedures. The top-level package, however, presents some assembly hurdles. First, many of these stacked packages experience warpage problems whereby the bottom package warps downward and the top package warps upward. This may result in stretched or broken solder joints. In most cases, however, this can be rectified through the use of low-warpage mold compounds. Second, the assembly method of the top package presents challenges related to stress reduction and long-term reliability. The most commonly used attachment method for the level-two package is a tacky flux dip, where the spheres are dipped into a tacky flux prior to component placement. This offers the flux action necessary to form the solder joint during reflow, but device support and protection can be less than adequate. Early evaluations, however, indicate that epoxy flux materials offer the top level device support and reliability enhancement required for these packages. In a recent analysis of PoP top-level attachment mechanisms, four materials were studied: tacky flux A (no-clean), tacky flux B (no-clean), a SAC 305 solder paste (type IV powder with 80% metal loading), and an epoxy flux. The devices were then subjected to drop testing and initial results indicate that epoxy flux offers the most robust performance with the most number of drops before the first failure (Figure 5). This implies that the dual function of this material ??? flux for solder joint formation and epoxy for bump encapsulation ??? delivers better performance than flux alone. As with tacky flux processes, when using epoxy flux, manufacturers dip the bottom-side spheres of the top-level component into the material prior to component placement. When the device travels through reflow, the solder joint is formed and each individual sphere is encapsulated with epoxy for an added level of protection (Figure 6).

Figure 6. Epoxy flux on a level two PoP device.

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Large Footprint BGA and CSP Devices

Epoxy fluxes are also delivering cost-efficiencies for traditional assembly operations, particularly in the case of large-format BGA and CSP devices. With larger devices ??? generally in the range of 23 ?? 23 mm or more ??? traditional underfill techniques require increased volumes of material to be dispensed to completely cover the device area. In addition, flow rates and cure times for such large volumes of standard underfill may adversely affect throughput rates and negatively impact units-per-hour (UPH). Epoxy flux methods allow production specialists to process these large devices in-line, while eliminating the need for dedicated dispensing equipment, cure ovens, and the time required for these additional process steps.

Versatility Beyond Traditional Underfill

New package configurations, finer pitches, and the need for ever increasing throughput rates are pushing current underfill systems to their limit. Of course, there will always be a place for traditional capillary underfills as well as the newer class of cornerbond and edgebond alternatives. But, for stacked packages, large footprint array devices, and many other emerging technologies, older material systems can’t offer the in-line processing advantages in tandem with the high level of reliability required for these new products.

Next-generation epoxy flux materials provide UPH, performance, and reliability required for high-volume manufacturing, while offering a level of versatility heretofore unavailable. With a dual-function flux and underfill in one material, epoxy fluxes have a broad application range for both packaging and board assembly environments. With capability for ball attach, PoP assembly, large-area-array device assembly and protection and much more, manufacturing firms can conceivably source one material for production of various products. And, because the material may be applied via dispensing, screen printing, jetting, or dipping, manufacturing flexibility is unprecedented.

Conclusion

The pace of package development is tremendous. Consumers continue to demand higher functioning, low-cost products and manufacturers must keep pace. High-volume, high-reliability solutions are the answer for optimization of production environments, and innovative underfill materials technology enablesthese advances.

References

B.Toleno, “Underfill Technology Developments”, SMT, May 2008.

G. Carson and M. Todd, “Underfill Technology: From Current to Next-generation Materials”, Advanced Packaging, June 2006.